linux-kernel - Re: [PATCH 00/10] steal tasks to improve CPU utilization

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Message-ID: <20181022170421.GF3117@worktop.programming.kicks-ass.net>
Date:   Mon, 22 Oct 2018 19:04:21 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Steve Sistare <steven.sistare@...cle.com>
Cc:     mingo@...hat.com, subhra.mazumdar@...cle.com,
        dhaval.giani@...cle.com, rohit.k.jain@...cle.com,
        daniel.m.jordan@...cle.com, pavel.tatashin@...rosoft.com,
        matt@...eblueprint.co.uk, umgwanakikbuti@...il.com,
        riel@...hat.com, jbacik@...com, juri.lelli@...hat.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 00/10] steal tasks to improve CPU utilization

On Mon, Oct 22, 2018 at 07:59:31AM -0700, Steve Sistare wrote:
> When a CPU has no more CFS tasks to run, and idle_balance() fails to
> find a task, then attempt to steal a task from an overloaded CPU in the
> same LLC. Maintain and use a bitmap of overloaded CPUs to efficiently
> identify candidates.  To minimize search time, steal the first migratable
> task that is found when the bitmap is traversed.  For fairness, search
> for migratable tasks on an overloaded CPU in order of next to run.
> 
> This simple stealing yields a higher CPU utilization than idle_balance()
> alone, because the search is cheap, so it may be called every time the CPU
> is about to go idle.  idle_balance() does more work because it searches
> widely for the busiest queue, so to limit its CPU consumption, it declines
> to search if the system is too busy.  Simple stealing does not offload the
> globally busiest queue, but it is much better than running nothing at all.

Why I don't dislike the idea; I feel it is unfortunate to have two
different mechanisms to do effectively the same thing.

Can't we improve idle_balance() instead of building this parallel
functionality?